Ink jet printers operate by ejecting ink onto a receiving substrate in controlled patterns of closely spaced ink droplets. By selectively regulating the pattern of ink droplets, ink jet printers can be used to produce a wide variety of printed materials, including text, graphics, images, and the like. Moreover, ink jet printers are capable of recording permanent images on a wide variety of substrates, including light reflective substrates such as various types and grades of paper, and light transmissive substrates such as transparencies.
Ink jet printers designed for use with various types of inks are well known in the art. Solvent-based inks, including both aqueous and non-aqueous, inks are well known. Images are formed by application of solvent-based inks to a receiving surface and subsequent removal, such as by evaporation or diffusion, of the solvent. Although solvent-based ink printing systems are suitable for some applications, their application is limited, primarily because the solvent tends to migrate into porous surfaces, thereby reducing the resolution of the printed image. Moreover, clogging of ink jet orifices as a result of solvent evaporation is a serious problem in solvent-based ink jet systems.
Phase change inks are solid at ambient temperatures and liquid at the elevated operating temperatures of an ink jet printing device. Ink jet droplets in the liquid phase are ejected from the printing device at an elevated operating temperature and rapidly solidify when they contact with the surface of a substrate to form the predetermined pattern. Phase change ink is advantageous for a variety of reasons. Problems associated with ink jet clogging resulting from solvent evaporation are largely eliminated, thereby improving the reliability of ink jet printing. Because the ink droplets solidify rapidly upon contact with the substrate, migration of ink along the printing medium is substantially reduced, and image quality and resolution is therefore substantially improved. Rapid solidification of phase change inks permits high quality images to be printed on a wide variety of porous and nonporous printing substrates.
Early references to phase change inks for ink jet printing involved monochrome inks jetted by electrostatic printing devices. Thus, for example, U.S. Pat. No. 3,653,932 discloses a low melting point (30.degree. C. to 50.degree. C.) ink having a base comprising di-esters of sebacic acid. In a similar process, U.S. Pat. No. 3,715,219 describes low melting point (30.degree. C. to 60.degree. C.) inks including a paraffin alcohol-based ink. One disadvantage of printing with low melting point phase change inks is that they are susceptible to softening and tend to exhibit offset problems. Specifically, when substrates printed with low melting point phase change inks are stacked or placed adjacent another surface, the ink tends to adhere to adjacent surfaces, particularly if the printed substrates are exposed to high temperatures.
U.S. Pat. Nos. 4,390,369 and 4,484,948 describe methods for producing monochrome phase change inks that employ a natural wax ink base, such as Japan wax, candelilla wax, and carnauba wax, which are printed using a drop-on-demand ink jet device at a temperature ranging between 65.degree. C. and 75.degree. C. U.S. Pat. No. 4,659,383 discloses a monochrome ink composition having an ink base including a C20-24 acid or alcohol, a ketone, and an acrylic resin plasticizer. These monochrome ink compositions are generally not durable and, when printed, become smudged upon routine handling and folding. U.S. Pat. No. 4,684,956 discloses phase change inks utilizing synthetic microcrystalline wax (hydrocarbon wax) and microcrystalline polyethylene wax. This molten composition can be applied to a variety of porous and non-porous substrates using drop-on-demand ink jet application techniques.
Color phase change ink jet systems have also been developed. Color ink jet printers typically utilize three primary color inks, in addition to black, that can provide a large spectrum of intermediate colors. Subtractive color mixing techniques utilizing cyan, magenta and yellow as primary colors are typically employed. European Patent Application Nos. 0187352 and 0206286 disclose phase change ink jet printing in color. The base inks for these systems include fatty acids, a thermoplastic polyethylene and a phase change material in the first application; and the alcohol portion of a thermosetting resin pair, a mixture of organic solvents (o- and p-toluene sulfonamide) and a dye in the second application.
Although ink jet printing using phase change inks generally produces high quality printing on light reflective substrates, ink jet printing of colored inks onto light transmissive media for displaying color images by overhead projection has been problematic. Special coatings are generally provided on light transmissive media to absorb solvent when solvent-based ink systems are used. See U.S. Pat. Nos. 4,503,111, 4,547,405, and 4,555,437. The development of phase change inks that are substantially transparent provides improved capability to print images on light transmissive substrates without requiring the use of special coatings. Phase change ink compositions disclosed in U.S. Pat. No. 4,899,761 are exemplary. As a consequence of the three-dimensional configuration of phase change ink droplets, however, phase change ink images projected by overhead projection generally do not exhibit high color saturation and clarity and may require reorientation after printing.
PCT Patent Application No. WO 88/08788 is directed to a method of treating transparencies printed with curved, light scattering ink droplets to improve their projection qualities. The printed ink droplets are overlaid with a transparent layer having an index of refraction that is substantially the same as the index of refraction of the ink droplets. Preferred coating materials include transparent polyurethane and acrylic. In this manner, the refractive effect of the curvature of the ink droplets is reduced.
U.S. Pat. No. 4,992,304 discloses a system for printing phase change ink on light transmissive substrates such as transparencies wherein an adhesion promoter layer is interposed between the substrate and ink layer. Adhesion promoter layers comprise a thermoplastic material, such as a thermoplastic polyamide.
In many ink jet printing systems, ink is printed directly onto the surface of the final receiving substrate. An ink jet printing system wherein an image is printed on an intermediate image transfer surface and subsequently transferred to the final receiving substrate is disclosed in U.S. Pat. No. 4,538,156 to Durkee et al. Inks having a polyhydric alcohol base colored with dyes that do not wet the surface of the intermediate transfer drum are disclosed for use with the ink jet printing system disclosed in the '156 patent.
U.S. Pat. Nos. 4,731,647 and 4,833,530 to Kohsahi disclose a system wherein a solvent is deposited on colorant to dissolve the colorant and form a transferable ink drop. The colorants and solvent are deposited directly onto paper or plastic colorant transfer sheets to form transferable ink droplets. The transferable drops are then contact transferred to a final receiving substrate, such as paper.
U.S. Pat. No. 5,099,256 to Anderson describes an ink jet printing system wherein ink is printed onto the surface of a thermally conductive intermediate drum. The intermediate drum surface is coated with a suitable film-forming silicone polymer allegedly having a high surface energy and high degree of surface roughness to prevent movement of the ink droplets after they have been applied to the intermediate surface. The drum surface is heated to dehydrate the ink droplets prior to transfer to the recording medium.
U.S. Pat. No. 4,743,920 discloses a thermal transfer recording system wherein ink forming a surface layer on an ink roll is selectively heated or softened to transfer ink onto a recording medium. Supercoolable, heat-transferable inks having a heat-fusible binder are used in this system. The ink binders are obtained by mixing supercoolable substances such as plasticizers with conventional heat-fusible binders such as thermoplastic resins, amide resins, natural or synthetic waxes, or the like.
U.S. Pat. No. 4,673,303 to Sansone et al. discloses an offset ink jet postage printing method and apparatus in which an inking roll applies ink to the first region of a dye plate. A lubricating hydrophilic oil is applied to the exterior surface of the printing drum or roll to facilitate transfer of printed images from the intermediate drum onto the receiving surface. The '303 patent also suggests that the ink can be modified to increase its viscosity after it has been applied to the surface of the drum upon exposure to electromagnetic radiation (visible or UV) or heat, or upon addition of a catalyst.
U.S. Pat. No. 5,087,603 relates to self-cross-linking aqueous resin dispersions obtained by emulsion-polymerizing a monomer composition in an aqueous medium. Cross-linking occurs upon evaporation of volatile components to produce a film coating having improved adhesive properties, water and solvent resistance, and durability.
U.S. Pat. No. 4,421,816 discloses a dry transfer decal in which a carrier layer is formed by application of mutually cross-linkable liquid prepolymers to a base sheet. The carrier layer is cross-linked by the action of heat or time, and ink layers are subsequently printed on the carrier coat in the desired decal pattern. Upon application of the decal to a receiving surface, the carrier film is exposed and protects the underlying ink layers from abrasion and degradation by exposure to solvents and the like.
U.S. Pat. No. 4,454,179 discloses a dry transfer article in which the ink component comprises a solvent-based ink, multi-component reactive ink, or actinic radiation curable ink. In multi-component reactive systems, reactive components are dissolved or dispersed in a suitable liquid medium, printed, solvent evaporated and then cured by reaction of the reactive components. Multi-component reactive ink systems involving combination of a reactive polyol resin and polyisocyanate to produce polyurethane inks are disclosed. Moreover, the '179 patent discloses that actinic radiation curable ink systems entail use of reactive prepolymers and monomers such as urethane acrylates responsive to actinic radiation (generally UV light) to effect curing.
Although many ink compositions and printing systems have been developed for various applications for electronic computer driven printers, the demand for increasingly higher resolution images and faster printing times, and the ability to print images on a variety of substrates, requires yet more refined ink compositions and ink jet printing systems. Specifically, there is a need for ink compositions and printing systems that can provide high resolution images on a variety of printing substrates at high printing rates. In addition to the foregoing requirements, it is also important that the printed ink image is flexible, durable and non-abradable.